Rollup merge of #97720 - cjgillot:all-fresh, r=petrochenkov

Always create elided lifetime parameters for functions

Anonymous and elided lifetimes in functions are sometimes (async fns) --and sometimes not (regular fns)-- desugared to implicit generic parameters.

This difference of treatment makes it some downstream analyses more complicated to handle.  This step is a pre-requisite to perform lifetime elision resolution on AST.

There is currently an inconsistency in the treatment of argument-position impl-trait for functions and async fns:
```rust
trait Foo<'a> {}
fn foo(t: impl Foo<'_>) {} //~ ERROR missing lifetime specifier
async fn async_foo(t: impl Foo<'_>) {} //~ OK
fn bar(t: impl Iterator<Item = &'_ u8>) {} //~ ERROR missing lifetime specifier
async fn async_bar(t: impl Iterator<Item = &'_ u8>) {} //~ OK
```

The current implementation reports "missing lifetime specifier" on `foo`, but **accepts it** in `async_foo`.
This PR **proposes to accept** the anonymous lifetime in both cases as an extra generic lifetime parameter.
This change would be insta-stable, so let's ping t-lang.
Anonymous lifetimes in GAT bindings keep being forbidden:
```rust
fn foo(t: impl Foo<Assoc<'_> = Bar<'_>>) {}
                         ^^        ^^
                       forbidden   ok
```
I started a discussion here: https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Anonymous.20lifetimes.20in.20universal.20impl-trait/near/284968606

r? ``@petrochenkov``

compiler/rustc_feature/src/active.rs

@@ -148,6 +148,8 @@ declare_features! (
     /// below (it has to be checked before expansion possibly makes
     /// macros disappear).
     (active, allow_internal_unstable, "1.0.0", None, None),
+    /// Allows using anonymous lifetimes in argument-position impl-trait.
+    (active, anonymous_lifetime_in_impl_trait, "1.63.0", None, None),
     /// Allows identifying the `compiler_builtins` crate.
     (active, compiler_builtins, "1.13.0", None, None),
     /// Outputs useful `assert!` messages

compiler/rustc_resolve/src/late.rs

@@ -758,7 +758,10 @@ impl<'a: 'ast, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
                 // We don't need to deal with patterns in parameters, because
                 // they are not possible for foreign or bodiless functions.
                 self.with_lifetime_rib(
-                    LifetimeRibKind::AnonymousPassThrough(fn_id, false),
+                    LifetimeRibKind::AnonymousCreateParameter {
+                        binder: fn_id,
+                        report_in_path: false,
+                    },
                     |this| walk_list!(this, visit_param, &sig.decl.inputs),
                 );
                 self.with_lifetime_rib(
@@ -792,18 +795,13 @@ impl<'a: 'ast, 'ast> Visitor<'ast> for LateResolutionVisitor<'a, '_, 'ast> {
                         // generic parameters.  This is especially useful for `async fn`, where
                         // these fresh generic parameters can be applied to the opaque `impl Trait`
                         // return type.
-                        let rib = if async_node_id.is_some() {
-                            // Only emit a hard error for `async fn`, since this kind of
-                            // elision has always been allowed in regular `fn`s.
+                        this.with_lifetime_rib(
                             LifetimeRibKind::AnonymousCreateParameter {
                                 binder: fn_id,
-                                report_in_path: true,
-                            }
-                        } else {
-                            LifetimeRibKind::AnonymousPassThrough(fn_id, false)
-                        };
-                        this.with_lifetime_rib(
-                            rib,
+                                // Only emit a hard error for `async fn`, since this kind of
+                                // elision has always been allowed in regular `fn`s.
+                                report_in_path: async_node_id.is_some(),
+                            },
                             // Add each argument to the rib.
                             |this| this.resolve_params(&declaration.inputs),
                         );

compiler/rustc_resolve/src/late/lifetimes.rs

@@ -1677,14 +1677,29 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                     break None;
                 }
 
-                Scope::Binder { ref lifetimes, scope_type, s, .. } => {
+                Scope::Binder { ref lifetimes, scope_type, s, where_bound_origin, .. } => {
                     if let Some(&def) = lifetimes.get(&region_def_id) {
                         break Some(def.shifted(late_depth));
                     }
                     match scope_type {
                         BinderScopeType::Normal => late_depth += 1,
                         BinderScopeType::Concatenating => {}
                     }
+                    // Fresh lifetimes in APIT used to be allowed in async fns and forbidden in
+                    // regular fns.
+                    if let Some(hir::PredicateOrigin::ImplTrait) = where_bound_origin
+                        && let hir::LifetimeName::Param(_, hir::ParamName::Fresh) = lifetime_ref.name
+                        && let hir::IsAsync::NotAsync = self.tcx.asyncness(lifetime_ref.hir_id.owner)
+                        && !self.tcx.features().anonymous_lifetime_in_impl_trait
+                    {
+                        rustc_session::parse::feature_err(
+                            &self.tcx.sess.parse_sess,
+                            sym::anonymous_lifetime_in_impl_trait,
+                            lifetime_ref.span,
+                            "anonymous lifetimes in `impl Trait` are unstable",
+                        ).emit();
+                        return;
+                    }
                     scope = s;
                 }
 

compiler/rustc_span/src/symbol.rs

@@ -341,6 +341,7 @@ symbols! {
         always,
         and,
         and_then,
+        anonymous_lifetime_in_impl_trait,
         any,
         append_const_msg,
         arbitrary_enum_discriminant,

src/test/ui-fulldeps/internal-lints/rustc_pass_by_value.rs

@@ -93,7 +93,7 @@ impl CustomStruct {
 
     fn test_alias(
         value: CustomAlias,
-        reference: &CustomAlias, //~ ERROR passing `CustomAlias<>` by reference
+        reference: &CustomAlias, //~ ERROR passing `CustomAlias<'_>` by reference
     ) {
     }
 }

src/test/ui-fulldeps/internal-lints/rustc_pass_by_value.stderr

@@ -94,11 +94,11 @@ error: passing `CustomStruct` by reference
 LL |         reference: &CustomStruct,
    |                    ^^^^^^^^^^^^^ help: try passing by value: `CustomStruct`
 
-error: passing `CustomAlias<>` by reference
+error: passing `CustomAlias<'_>` by reference
   --> $DIR/rustc_pass_by_value.rs:96:20
    |
 LL |         reference: &CustomAlias,
-   |                    ^^^^^^^^^^^^ help: try passing by value: `CustomAlias<>`
+   |                    ^^^^^^^^^^^^ help: try passing by value: `CustomAlias<'_>`
 
 error: passing `WithParameters<T, 1>` by reference
   --> $DIR/rustc_pass_by_value.rs:110:20

src/test/ui/generic-associated-types/bugs/issue-87748.stderr

@@ -4,12 +4,12 @@ error[E0478]: lifetime bound not satisfied
 LL |     fn do_sth(_: u32) {}
    |     ^^^^^^^^^^^^^^^^^
    |
-note: lifetime parameter instantiated with the anonymous lifetime #2 defined here
+note: lifetime parameter instantiated with the anonymous lifetime as defined here
   --> $DIR/issue-87748.rs:18:5
    |
 LL |     fn do_sth(_: u32) {}
    |     ^^^^^^^^^^^^^^^^^
-note: but lifetime parameter must outlive the anonymous lifetime #1 defined here
+note: but lifetime parameter must outlive the anonymous lifetime as defined here
   --> $DIR/issue-87748.rs:18:5
    |
 LL |     fn do_sth(_: u32) {}

src/test/ui/generic-associated-types/issue-95305.rs

@@ -3,15 +3,18 @@
 // at some point in the future.
 
 #![feature(generic_associated_types)]
-
+#![feature(anonymous_lifetime_in_impl_trait)]
 trait Foo {
     type Item<'a>;
 }
 
 fn foo(x: &impl Foo<Item<'_> = u32>) { }
                        //~^ ERROR `'_` cannot be used here [E0637]
 
+// Ok: the anonymous lifetime is bound to the function.
 fn bar(x: &impl for<'a> Foo<Item<'a> = &'_ u32>) { }
-                                      //~^ ERROR missing lifetime specifier
+
+// Ok: the anonymous lifetime is bound to the function.
+fn baz(x: &impl for<'a> Foo<Item<'a> = &u32>) { }
 
 fn main() {}

src/test/ui/generic-associated-types/issue-95305.stderr

@@ -4,18 +4,6 @@ error[E0637]: `'_` cannot be used here
 LL | fn foo(x: &impl Foo<Item<'_> = u32>) { }
    |                          ^^ `'_` is a reserved lifetime name
 
-error[E0106]: missing lifetime specifier
-  --> $DIR/issue-95305.rs:14:41
-   |
-LL | fn bar(x: &impl for<'a> Foo<Item<'a> = &'_ u32>) { }
-   |                                         ^^ expected named lifetime parameter
-   |
-help: consider using the `'a` lifetime
-   |
-LL | fn bar(x: &impl for<'a> Foo<Item<'a> = &'a u32>) { }
-   |                                         ~~
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error
 
-Some errors have detailed explanations: E0106, E0637.
-For more information about an error, try `rustc --explain E0106`.
+For more information about this error, try `rustc --explain E0637`.

src/test/ui/issues/issue-37884.stderr

@@ -6,7 +6,7 @@ LL |     fn next(&'a mut self) -> Option<Self::Item>
    |
    = note: expected fn pointer `fn(&mut RepeatMut<'a, T>) -> Option<_>`
               found fn pointer `fn(&'a mut RepeatMut<'a, T>) -> Option<_>`
-note: the anonymous lifetime #1 defined here...
+note: the anonymous lifetime as defined here...
   --> $DIR/issue-37884.rs:6:5
    |
 LL |     fn next(&'a mut self) -> Option<Self::Item>

src/test/ui/nll/ty-outlives/impl-trait-captures.stderr

@@ -2,14 +2,14 @@ error[E0700]: hidden type for `impl Trait` captures lifetime that does not appea
   --> $DIR/impl-trait-captures.rs:11:5
    |
 LL | fn foo<'a, T>(x: &T) -> impl Foo<'a> {
-   |                  -- hidden type `&ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrAnon(0)) T` captures the anonymous lifetime defined here
+   |                  -- hidden type `&ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrNamed(DefId(0:13 ~ impl_trait_captures[1afc]::foo::'_), '_)) T` captures the anonymous lifetime defined here
 LL |     x
    |     ^
    |
-help: to declare that the `impl Trait` captures `ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrAnon(0))`, you can add an explicit `ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrAnon(0))` lifetime bound
+help: to declare that the `impl Trait` captures `ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrNamed(DefId(0:13 ~ impl_trait_captures[1afc]::foo::'_), '_))`, you can add an explicit `ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrNamed(DefId(0:13 ~ impl_trait_captures[1afc]::foo::'_), '_))` lifetime bound
    |
-LL | fn foo<'a, T>(x: &T) -> impl Foo<'a> + ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrAnon(0)) {
-   |                                      ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+LL | fn foo<'a, T>(x: &T) -> impl Foo<'a> + ReFree(DefId(0:8 ~ impl_trait_captures[1afc]::foo), BrNamed(DefId(0:13 ~ impl_trait_captures[1afc]::foo::'_), '_)) {
+   |                                      ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
 error: aborting due to previous error
 

src/test/ui/suggestions/impl-trait-missing-lifetime-gated.rs

@@ -0,0 +1,21 @@
+// edition:2021
+// gate-test-anonymous_lifetime_in_impl_trait
+// Verify the behaviour of `feature(anonymous_lifetime_in_impl_trait)`.
+
+fn f(_: impl Iterator<Item = &'_ ()>) {}
+//~^ ERROR anonymous lifetimes in `impl Trait` are unstable
+
+fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+//~^ ERROR anonymous lifetimes in `impl Trait` are unstable
+//~| ERROR missing lifetime specifier
+
+// Anonymous lifetimes in async fn are already allowed.
+// This is understood as `fn foo<'_1>(_: impl Iterator<Item = &'_1 ()>) {}`.
+async fn h(_: impl Iterator<Item = &'_ ()>) {}
+
+// Anonymous lifetimes in async fn are already allowed.
+// But that lifetime does not participate in resolution.
+async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+//~^ ERROR missing lifetime specifier
+
+fn main() {}

src/test/ui/suggestions/impl-trait-missing-lifetime-gated.stderr

@@ -0,0 +1,44 @@
+error[E0658]: anonymous lifetimes in `impl Trait` are unstable
+  --> $DIR/impl-trait-missing-lifetime-gated.rs:5:31
+   |
+LL | fn f(_: impl Iterator<Item = &'_ ()>) {}
+   |                               ^^
+   |
+   = help: add `#![feature(anonymous_lifetime_in_impl_trait)]` to the crate attributes to enable
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/impl-trait-missing-lifetime-gated.rs:8:50
+   |
+LL | fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+   |                                                  ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'static ()> { x.next() }
+   |                                                  ~~~~~~~
+
+error[E0658]: anonymous lifetimes in `impl Trait` are unstable
+  --> $DIR/impl-trait-missing-lifetime-gated.rs:8:31
+   |
+LL | fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+   |                               ^^
+   |
+   = help: add `#![feature(anonymous_lifetime_in_impl_trait)]` to the crate attributes to enable
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/impl-trait-missing-lifetime-gated.rs:18:56
+   |
+LL | async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+   |                                                        ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'static ()> { x.next() }
+   |                                                        ~~~~~~~
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0106, E0658.
+For more information about an error, try `rustc --explain E0106`.

src/test/ui/suggestions/impl-trait-missing-lifetime.rs

@@ -1,2 +1,19 @@
-fn f(_: impl Iterator<Item = &'_ ()>) {} //~ ERROR missing lifetime specifier
+// edition:2021
+
+#![feature(anonymous_lifetime_in_impl_trait)]
+
+// This is understood as `fn foo<'_1>(_: impl Iterator<Item = &'_1 ()>) {}`.
+fn f(_: impl Iterator<Item = &'_ ()>) {}
+
+// But that lifetime does not participate in resolution.
+fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+//~^ ERROR missing lifetime specifier
+
+// This is understood as `fn foo<'_1>(_: impl Iterator<Item = &'_1 ()>) {}`.
+async fn h(_: impl Iterator<Item = &'_ ()>) {}
+
+// But that lifetime does not participate in resolution.
+async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+//~^ ERROR missing lifetime specifier
+
 fn main() {}

src/test/ui/suggestions/impl-trait-missing-lifetime.stderr

@@ -1,14 +1,27 @@
 error[E0106]: missing lifetime specifier
-  --> $DIR/impl-trait-missing-lifetime.rs:1:31
+  --> $DIR/impl-trait-missing-lifetime.rs:9:50
    |
-LL | fn f(_: impl Iterator<Item = &'_ ()>) {}
-   |                               ^^ expected named lifetime parameter
+LL | fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+   |                                                  ^^ expected named lifetime parameter
    |
-help: consider introducing a named lifetime parameter
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
    |
-LL | fn f<'a>(_: impl Iterator<Item = &'a ()>) {}
-   |     ++++                          ~~
+LL | fn g(x: impl Iterator<Item = &'_ ()>) -> Option<&'static ()> { x.next() }
+   |                                                  ~~~~~~~
 
-error: aborting due to previous error
+error[E0106]: missing lifetime specifier
+  --> $DIR/impl-trait-missing-lifetime.rs:16:56
+   |
+LL | async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'_ ()> { x.next() }
+   |                                                        ^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
+help: consider using the `'static` lifetime
+   |
+LL | async fn i(x: impl Iterator<Item = &'_ ()>) -> Option<&'static ()> { x.next() }
+   |                                                        ~~~~~~~
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0106`.

src/test/ui/wf/wf-in-foreign-fn-decls-issue-80468.stderr

@@ -21,11 +21,11 @@ note: because this has an unmet lifetime requirement
    |
 LL | pub struct Wrapper<T: Trait>(T);
    |                       ^^^^^ introduces a `'static` lifetime requirement
-note: the anonymous lifetime #1 defined here...
-  --> $DIR/wf-in-foreign-fn-decls-issue-80468.rs:16:5
+note: the anonymous lifetime as defined here...
+  --> $DIR/wf-in-foreign-fn-decls-issue-80468.rs:16:29
    |
 LL |     pub fn repro(_: Wrapper<Ref>);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                             ^^^
 note: ...does not necessarily outlive the static lifetime introduced by the compatible `impl`
   --> $DIR/wf-in-foreign-fn-decls-issue-80468.rs:13:1
    |

src/tools/clippy/clippy_lints/src/inherent_to_string.rs

@@ -2,7 +2,7 @@ use clippy_utils::diagnostics::span_lint_and_help;
 use clippy_utils::ty::{implements_trait, is_type_diagnostic_item};
 use clippy_utils::{get_trait_def_id, paths, return_ty, trait_ref_of_method};
 use if_chain::if_chain;
-use rustc_hir::{ImplItem, ImplItemKind};
+use rustc_hir::{GenericParamKind, ImplItem, ImplItemKind};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::sym;
@@ -102,7 +102,7 @@ impl<'tcx> LateLintPass<'tcx> for InherentToString {
             let decl = &signature.decl;
             if decl.implicit_self.has_implicit_self();
             if decl.inputs.len() == 1;
-            if impl_item.generics.params.is_empty();
+            if impl_item.generics.params.iter().all(|p| matches!(p.kind, GenericParamKind::Lifetime { .. }));
 
             // Check if return type is String
             if is_type_diagnostic_item(cx, return_ty(cx, impl_item.hir_id()), sym::String);

src/tools/clippy/clippy_lints/src/lifetimes.rs

@@ -9,8 +9,8 @@ use rustc_hir::intravisit::{
 use rustc_hir::FnRetTy::Return;
 use rustc_hir::{
     BareFnTy, BodyId, FnDecl, GenericArg, GenericBound, GenericParam, GenericParamKind, Generics, Impl, ImplItem,
-    ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, ParamName, PolyTraitRef, PredicateOrigin,
-    TraitBoundModifier, TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WherePredicate,
+    ImplItemKind, Item, ItemKind, LangItem, Lifetime, LifetimeName, LifetimeParamKind, ParamName, PolyTraitRef,
+    PredicateOrigin, TraitBoundModifier, TraitFn, TraitItem, TraitItemKind, Ty, TyKind, WherePredicate,
 };
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::hir::nested_filter as middle_nested_filter;
@@ -338,7 +338,10 @@ fn could_use_elision<'tcx>(
 fn allowed_lts_from(named_generics: &[GenericParam<'_>]) -> FxHashSet<RefLt> {
     let mut allowed_lts = FxHashSet::default();
     for par in named_generics.iter() {
-        if let GenericParamKind::Lifetime { .. } = par.kind {
+        if let GenericParamKind::Lifetime {
+            kind: LifetimeParamKind::Explicit,
+        } = par.kind
+        {
             allowed_lts.insert(RefLt::Named(par.name.ident().name));
         }
     }
@@ -379,6 +382,7 @@ impl<'a, 'tcx> RefVisitor<'a, 'tcx> {
                 self.lts.push(RefLt::Static);
             } else if let LifetimeName::Param(_, ParamName::Fresh) = lt.name {
                 // Fresh lifetimes generated should be ignored.
+                self.lts.push(RefLt::Unnamed);
             } else if lt.is_elided() {
                 self.lts.push(RefLt::Unnamed);
             } else {

src/tools/clippy/clippy_lints/src/ptr.rs

@@ -495,12 +495,13 @@ fn check_mut_from_ref<'tcx>(cx: &LateContext<'tcx>, sig: &FnSig<'_>, body: Optio
     if let FnRetTy::Return(ty) = sig.decl.output
         && let Some((out, Mutability::Mut, _)) = get_rptr_lm(ty)
     {
+        let out_region = cx.tcx.named_region(out.hir_id);
         let args: Option<Vec<_>> = sig
             .decl
             .inputs
             .iter()
             .filter_map(get_rptr_lm)
-            .filter(|&(lt, _, _)| lt.name == out.name)
+            .filter(|&(lt, _, _)| cx.tcx.named_region(lt.hir_id) == out_region)
             .map(|(_, mutability, span)| (mutability == Mutability::Not).then(|| span))
             .collect();
         if let Some(args) = args